Sister's Got Me Started, Sort Of

When i was a youngster I used to purposefully leave my fly unzipped. Sometimes my "whiteys" would show and sometimes I'd just be naked underneath until someone would tell me to zip it up. On one ocassion I had slipped on a pair of mom's panties, blue ones, thinking they wouldn't show. Well you know people have keen eyes sometimes and sure enough my older sister came over and whispered in my ear she could see the blue. She smiled and I smiled back after I caught myself. She then whispered come to my room later and I did.
She called me in and she had laid out several pairs of her panties on the bed. She told me I could pick and choose but there was a catch. I was anxious and getting a rise as well. What I asked? You have to go out on the front porch in plain sight of the street and put them on. After I thought a moment I agreed and she went with me to be sure I did it.
Once there I looked around and it seemed void of people going by. I started to turn away from the street adn she told me no dice. So I scanned around again and undid my jeans which fell away quickly. I hooked the blue panties and wriggled them down being embarressed as once again my hardness was half way there. She reached over and hung the her red ones there and smiled, go on. Boy did I as fast as I could. But not fast enough. Just as I'd stepped into on leg our usual mailman rounded the hedge and headed for the front steps. By the time he reached the first step I was wiggling for all my might to get them in place.
He was cool, maybe in his 50s I'd guess. He said Hi and looked at the two of us with my pants at half mast and just smiled, left the mail and left. My hardness was gone just like the mailman.
My sis told me I did good and promised that I could keep the red ones as long as I wanted to. But if I'd like to exchange them or maybe have one of her bras all I had to do was agree to another dare. I told her I'd think about it and I did. More later.